Method for measuring the conservation of stereophonic audio signals and method for identifying jointly coded stereophonic audio signals

ABSTRACT

Jointly processed stereophonic audio signal properties are identified using a stereophonic signal as reference signal and creating a signal for testing by processing the stereophonic signal, e.g. by coding and subsequently decoding it. Both signals are transformed into the frequency domain to create representative spectral data for the respective subbands. Correlation coefficients are determined for each subband both of the reference signal and also of the signal for testing on the basis of the spectral data of the channels of the reference signal or of the signal for testing. From the comparison of the correlation coefficients belonging to the same subband, jointly processed stereophonic audio signals are detected if at least one of the correlation coefficients of the signal for testing greatly exceeds the correlation coefficient of the reference signal for the same subband.

FIELD OF THE INVENTION

According to a first aspect, the present application refers to a methodfor measuring the conservation of stereophonic audio signals whenemploying a processing technique or a coding technique on stereophonicsignals.

According to a further aspect, the present invention refers to a methodfor identifying jointly processed or coded stereophonic audio signals.

DESCRIPTION OF THE PRIOR ART

For the purposes of data reduction, techniques for the joint stereocoding of stereophonic audio signals are being used increasingly in thecase of very high compression factors. A known coding technique of thistype is the so-called "intensity stereo technique". When employing theintensity stereo technique, audible distortions may occur in the stereoacoustic pattern. It is therefore of interest to make it possible todetect such distortions by means of measurement. It is also of interestto establish whether a coded and then decoded signal was coded using ajoint stereo coding technique.

From the prior, non-prepublished German patent application P 43 31376.0-31, a method for determining the type of coding to be chosen forcoding at least two signals is known, in which a transformation ofsignals into the frequency domain is performed and, on the basis ofspectral values, a similarity measure is determined, on the basis ofwhich the type of coding to be chosen is specified. Here, one of thesignals in a coding type which is used if a high similarity measure isdetected and which is e.g. the intensity stereo coding, is first codedand then decoded to create a coding-error-afflicted signal. Both thissignal and the original signal, which is not afflicted with the codingerror, are transformed into the frequency domain. Spectral values of therespective corresponding channels, e.g. of the left or e.g. of the rightchannel in the case of a twin-channel stereo signal, of mutuallycorresponding subbands both of the transformed coding-error-afflictedsignal and of the transformed coding-error-free signal are compared withone another employing a masked hearing threshold, where the maskedhearing threshold is determined by psychoacoustic calculations. Thiscomparison of the spectral values of the respective correspondingchannels forms the basis for the determination of a similarity measure,on the basis of which the coding type to be chosen is specified.

A known measurement method for the hearing-related evaluation ofdistortions in the stereo acoustic pattern is the so-called NMR method,which is known from the following prepublication: K. Brandenburg, T.Sporer: "`NMR` and `masking flag`: Evaluation of Quality usingPerceptual Criteria", Proc. of the 11th International AES Conference onAudio Test and Measurement, Portland 1992 pp. 169-179. However, it isnot possible to record the stereophonic acoustic pattern with a methodof this kind.

Furthermore, with the known methods described above it is not possibleto determine whether the coding technique used is a joint stereo codingone.

From WO-A-8908357 a method for the quantitative real-time recording ofthe audibility of disturbances in the coding of audio signals is known,in which a processed audio signal which is to be compared, obtained e.g.by coding and subsequent decoding, is correlated with the originalsignal in order to establish the signal delay of a system processing thesignal, whereupon the difference of the signals for comparison is formedin the time domain, taking into account the ascertained signal delaytime. The spectral composition both of the original signal and also ofthe difference signal is then formed. From the spectral composition ofthe original signal, the masked hearing threshold of the human ear isdetermined and compared with the spectral composition of the differencesignal. The spectral regions of the difference signal which exceed themasked hearing threshold serve for the quantitative recording of theaudibility of disturbances in the signal processing.

SUMMARY OF THE INVENTION

It is an object of the present invention to disclose a method foridentifying jointly coded stereophonic audio signals.

This object is achieved by a method for identifying jointly processedstereophonic audio signals, with the following steps:

providing a stereophonic signal as reference signal;

processing the stereophonic signal by means of the processing techniqueto provide a signal for testing derived from this stereophonic signal;

transforming the reference signal and the signal for testing into thefrequency domain to create representative spectral data for therespective subbands;

determining correlation coefficients for each subband of the referencesignal, in each case on the basis of the spectral data of the channelsof the reference signal;

determining correlation coefficients for each subband of the signal fortesting, in each case on the basis of the spectral data of the channelsof the signal for testing;

comparing the correlation coefficients belonging to the same subband;and

detecting jointly processed stereophonic audio signals if at least oneof the correlation coefficients of the signal for testing greatlyexceeds the correlation coefficient of the reference signal for the samesubband.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the methods according to the present inventionwill be described in more detail below making reference to the encloseddrawing, in which:

The single FIGURE shows a block diagram of a circuit for implementingthe methods according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The method according to the first invention aspect now to be describedserves to measure the conservation of stereophonic audio signalproperties when using a coding technique on stereophonic signals andserves in particular to measure the conservation of psychoacousticquantities which are important for an undisturbed stereo acousticpattern when using a joint stereo coding technique.

A signal for testing X' with the channels L' and R' is formed on thebasis of a stereophonic signal X with the channels L and R by coding andsubsequent decoding. In the preferred embodiment, a joint stereo codingtechnique, preferably the intensity stereo coding technique, is used.The signal for testing X' and the reference signal X are fed, indelay-compensated form, to inputs of representation circuits 1, whichperform a time/frequency representation of the signal for testing X' andthe reference signal X. Such representation circuits 1 are known per sein the prior art and can be implemented by means of a filter bank or bymeans of a transformation circuit with subsequent grouping of the outputvalues. In the preferred application of the FFT (the fast Fouriertransformation), the output quantities of the representation circuits 1are the respective spectral data for the respective subbands.

In a function block 2a, signal quantities Gi are formed for each subbandi of the reference signal X, in each case on the basis of the spectraldata of the right and left channels R, L of the reference signal. In acorresponding function block 2b, signal quantities Gi' are formed foreach subband i of the signal for testing X', in each case on the basisof the spectral data of the left and right channels L', R'. In anevaluation block 3, to which the signal quantities Gi', G^(i) are fed,the signal quantities Gi, Gi' belonging to the same subband i arecompared with one another in each case. From this comparison, as will beclarified in detail below, conclusions are drawn as to the conservationof the stereophonic audio signal properties or the disturbance of thestereo acoustic pattern for the coding technique being used.

The size of the subbands, which are specified by the representationcircuits 1, is preferably chosen according to the frequency resolutionof the human auditory system or according to the so-called "Bark" scale.

For the purposes of the method according to the present invention, theuse of different signal quantities is conceivable, among them thespecial cases, explained hereafter, of the use of the correlationcoefficients and the level differences. Every type of signal quantitycan be considered, insofar as it is formed, on the one hand, for eachsubband of the reference signal, in each case on the basis of thespectral data of the individual channels of the reference signal, and,on the other hand, for each subband of the signal for testing, in eachcase on the basis of the spectral data of the channels of the signal fortesting.

In the preferred embodiment the signal quantities Gi, Gi' comprise thecorrelation coefficient k_(i), which specifies the correlation of thespectral data of the individual channels for the respective subbands i,on the one hand for the signal for testing X' and, on the other hand,for the reference signal X.

In the special case of the use of a stereophonic signal with 2 channelsl, r, the correlation coefficient k_(i) for each subband i is given bythe following equation: ##EQU1##

In the equation for the calculation of the correlation coefficient k_(i)and in the equation for the calculation of the level difference dL_(i),l_(i;j) and r_(i;j) designate the jth temporal spectral value of the ithsubband in the left and right channel.

Furthermore, the signal quantities Gi, Gi' comprise the leveldifferences dL_(i), i.e. the differences in the levels of the spectraldata of the left and right channels for the respective subbands i bothfor the reference signal X and for the signal for testing X'. The leveldifferences are determined by the following equation: ##EQU2##

The above formulae for the correlation coefficients and the leveldifferences are valid in the case where the representation circuits 1are implemented as a filter bank.

The evaluation block 3 assesses the conservation of the stereophonicaudio signal properties by comparing the signal quantities belonging tothe same subband i. If the level differences dL_(i) ', dL_(i) for thesignal for testing X' and the reference signal X differ, it can beconcluded that there is impairment of the local representation in thetest signal or a disturbance of the stereo acoustic pattern due to thecoding method being used.

If the correlation coefficient k_(i) of the signal for testing X' isconsiderably higher for a subband i than the corresponding correlationcoefficient k_(i) for the same subband of the reference signal X, it canbe concluded that a joint stereo coding technique, in particular that ofintensity stereo coding, has been used.

We claim:
 1. Method for identifying jointly processed stereophonic audiosignals, comprising the steps of:providing a stereophonic signal asreference signal; processing the stereophonic signal by means of theprocessing technique to provide a signal for testing derived from thisstereophonic signal; transforming the reference signal and the signalfor testing into the frequency domain to create representative spectraldata for the respective subbands; determining correlation coefficientsfor each subband of the reference signal, in each case on the basis ofthe spectral data of the channels of the reference signal; determiningcorrelation coefficients for each subband of the signal for testing, ineach case on the basis of the spectral data of the channels of thesignal for testing; comparing the correlation coefficients belonging tothe same subband; and detecting jointly processed stereophonic audiosignals if at least one of the correlation coefficients of the signalfor testing greatly exceeds the correlation coefficient of the referencesignal for the same subband.
 2. Method according to claim 1, wherein thestep of processing the stereophonic signal by means of the processingtechnique to provide a signal for testing derived from this stereophonicsignal comprises the following step:coding and subsequent decoding ofthe stereophonic signal by means of the coding technique to provide thesignal for testing.
 3. Method according to claim 1 or 2, characterizedin that jointly coded stereophonic audio signals are identified if atleast one of the correlation coefficients of the signal for testingexceeds the correlation coefficient of the reference signal for the samesubband by at least 0,25.
 4. Method according to claim 1, wherein thestereophonic signal has two channels andfor which the correlationcoefficients for each subband are given by the following equation:##EQU3## where l_(i;j) and r_(i;j) designate the jth temporal spectralvalue of the ith subband in the left or right channel.